The present invention relates to an active matrix type liquid crystal display.
In usual liquid crystal displays, an electric field is applied to liquid crystal molecules in a liquid crystal layer held between one pair of substrates to change a direction of orientation of the liquid crystal molecules, and thereby caused change in the optical characteristics of the liquid crystal layer is utilized for making display.
Among the prior active driving type liquid crystal displays, typical is the Twisted Nematic (TN) Display mode in which an electrode is provided on each of the two substrates holding a liquid crystal between themselves, the direction of electric field applied to the liquid crystal is made roughly perpendicular to the substrate interface, and the optical rotatory power of the liquid crystal is utilized for making display. This TN type liquid crystal display has a fault that the viewing angle is narrow.
Thus, there has been proposed the in-plane switching (IPS) mode in which an interdigital electrode is used to make the generated electric field have a component roughly parallel to the substrate surface, and the liquid crystal is rotated nearly within a plane, and a birefringence of liquid crystal is used for making display (for example, JP-A-6-202127 and JP-A-6-160878). This IPS mode has a merit that it is based on the in-plane switching of liquid crystal molecules so that it has a wider viewing angle as compared with the prior TN mode and at the same time it has a lower storage capacitance. Thus, IPS is considered hopeful and able to replace the prior TN liquid crystal displays, and is making a rapid progress in the recent years. Further, an IPS mode in which either one electrode to which electric field is applied is constructed from a transparent electrically conductive film and thereby transmittance is improved has also been proposed (JP-A-9-73101). Such liquid crystal displays excellent in viewing angle characteristics (contrast ratio, gray scale reversal) and high in brightness are potent techniques aiming at monitors and televisions of wide display region.
When a high-resolution is to be given to liquid crystal display, a higher precision of overlapping is required between the substrate surface on which thin film transistor (TFT) is formed (active matrix substrate) and the substrate surface on which color filter layer (CF layer) is formed. A decrease in the precision of overlapping, namely a decrease in the precision of the alignment between black matrix and scanning electrode wirings and image signal wirings, causes a drop in a practical aperture ratio, and further causes a decrease in the contrast ratio due to exposure of the originally unseen domains in the light-screened region, which brings about a decrease in the performance of high-resolution display. Thus, in the prior vertical TN mode, a technique of taking CF layer and light-shielding black matrix (BM) into the TFT substrate side to improve the allowance in the precision of overlapping greatly has been developed, as shown in JP-A-4-253028, etc.
Further, a technique of applying the technique of taking such CF layer onto active matrix substrate so as to secure an allowance of overlapping between the up and down substrates to the IPS lateral electric field mode has also been proposed (JP-A-11-190856).
Further, a structure of holding a pixel electrode for driving a liquid crystal and a common electrode between the liquid crystal layer and the CF layer to suppress the practical rise in the driving voltage of liquid crystal has been proposed (for example, JP-A-2000-111957).
However, among the techniques mentioned above, the technique of JP-A-11-190856 has a problem that a driving voltage of liquid crystal is remarkably increased, even though the relaxation of after image can be accelerated and the occurrence of defective display due to after image can be suppressed when CF layer is formed as a part of the upper layer insulating film of a pixel electrode and common electrode.
On the other hand, according to the technique of JP-A-2000-111957, the relaxation time of after image becomes longer even though the rise in driving voltage of liquid crystal can be suppressed, and therefore this technique has a problem in the point of suppression of after image as a displaying characteristic. In cases where CF layer or BM layer is formed on an active matrix substrate, nothing screens the reflected light from pixel electrode or common electrode for driving the liquid crystal in IPS method, so that when the liquid crystal display is viewed from the front side, the contrast ratio decreases due to the reflection from the electrode surfaces.
The object of the present invention is to provide a liquid crystal display of high image quality which can be driven at a low voltage and shows a lowered inhomogeneity of display caused by an after image phenomenon.
In order to solve the problem mentioned above, the liquid crystal display of the present invention comprises one pair of substrates which are a first substrate and a second substrate wherein at least one of the substrates is transparent, a liquid crystal layer and a color filter layer both provided between said one pair of substrates, a plurality of thin film transistors which are provided on the first substrate existing on a downside of the color filter layer and are connected to an image signal wiring and a scanning signal wiring, a common electrode giving a standard potential, and a pixel electrode connected to the thin film transistors and placed opposite to the common electrode in a pixel region,
wherein said common electrode and said pixel electrode are placed in different layers from each other through an interlaminar insulating film in the form of layer comprising at least two layers comprising said color filter layer and an orientation direction of liquid crystal molecules of the liquid crystal layer is controlled by a voltage applied through the interlaminar insulating film and between the common electrode and the pixel electrode to make display.
According to the construction mentioned above, the pixel electrode and the common electrode are placed in different layers from each other sandwiching the interlaminar insulating film comprising the color filter layer, and therefore the electric field for driving the liquid crystal molecules through the pixel electrodes and common electrode is given to the liquid crystal layer through intermediation of the interlaminar insulating film. Accordingly, the electric charges which are generated due to polarization, etc. in the liquid crystal layer, interlaminar insulating layer, alignment layer present in the one pair of substrates and the interfaces thereof and accumulated can rapidly be relaxed. Further, since the pixel electrode gives an electric field to the liquid crystal layer through the interlaminar insulating layer more readily than the common electrode, the density of electric field in the edge region of electrodes in which electric field is apt to be concentrated can be relaxed effectively. As its result, after image characteristics can be improved, and a high quality liquid display showing a lowered inhomogeneity of display caused by an after image phenomenon can be realized.
Further, since a part of the interlaminar insulating film existing between the image element electrode and common electrode is replaced with a color filter layer having a relatively large dielectric constant, an effective electric field can be supplied to the liquid crystal layer, so that the driving voltage for driving the liquid crystal can be reduced as compared with the case of using a general organic insulating material.
More concretely speaking, the liquid crystal display comprises one pair of substrates which are a first substrate and a second substrate wherein at least one of the substrate is transparent, and a liquid crystal layer and a color filter layer both provided between said one pair of substrates, wherein said color filter layer is placed close to the first substrate and the liquid crystal layer is placed between the color filter layer and the second substrate,
wherein a plurality of scanning signal wirings, a plurality of image signal wirings and a plurality of transistors connected to the image signal wirings and the scanning signal wirings are placed on the first substrate existing on a downside of the color filter layer,
each region surrounded by the plurality of scanning signal wirings and the image signal wirings constitutes at least one pixel, and each pixel is provided with a common electrode connected with a common electrode wiring over a plurality of pixels to give a standard potential and a pixel electrode connected to the transistor and placed opposite to the common electrode in a pixel region,
wherein said common electrode and said pixel electrode are placed in different layers from each other through an interlaminar insulating film in the form of layer comprising at least two layers comprising said color filter layer and an orientation direction of liquid crystal molecules of the liquid crystal layer is controlled by a voltage applied through the interlaminar insulating film and between the common electrode and the pixel electrode to make display.
In constructing the above-mentioned liquid crystal display, the following elements may be added.
(1) Said common electrode coats at least a part of the image signal wiring and the scanning signal wiring through an insulating film.
(2) Said insulating film is an interlaminar insulating film comprising at least two layers.
(3) At least one layer of said interlaminar insulating film is made of an organic substance.
(4) An insulating overcoat layer is provided in a boundary part between two kinds of color filter layers on the image signal wiring or the scanning signal wiring, and the common electrode is formed on said overcoat layer.
(5) An overcoat layer for protecting the color filter is provided on an upside of the color filter layer, and the common electrode is formed on the overcoat layer.
(6) The common electrode or the common electrode wiring is formed lattice-wise so as to surround the pixel.
(7) An overcoat layer for protecting the color filter is provided on an upside of the color filter layer, and the pixel electrode is formed on the overcoat layer.
(8) Said overcoat layer or said interlaminar insulating film is made of a photosensitive resin.
(9) At least one member selected from the group consisting of the pixel electrode and the common electrode is constituted of a transparent electrode.
(10) Said transparent electrode is constituted of an ion doped titanium oxide film or an ion doped zinc oxide (ZnO) film.
(11) Said common electrode or said common electrode wiring is made of an alloy containing at least one member selected from the group consisting of Al, Cr, Mo, Ta and W.
(12) An antireflection layer is formed on an upside of the common electrode or the common electrode wiring.
(13) As said antireflection layer, a film containing a black-colored pigment is formed.
(14) As said antireflection layer, a phase difference film is laminated.
(15) As said antireflection layer, the common electrode or the common electrode wiring is formed into a laminated structure containing a magnetic material.
(16) The orientation direction of the liquid crystal molecules at two interfaces between the liquid crystal layer and alignment layers formed on said one pair of substrates are roughly in the same direction.
(17) At least one of the alignment layers formed on said one pair of substrates is a photo-reactive material layer. The alignment layers can be formed by irradiating a roughly linearly polarized light to the light-reflecting material layer.
(18) A pre tilt angle of the liquid crystal layer is 5xc2x0 (5 degrees) or less.